Known is an inverted pendulum type vehicle or an omni-directional vehicle incorporated with a frictional drive device which comprises a pair of drive assemblies individually actuated by electric motors and a main wheel held between the drive assemblies and frictionally driven by the drive assemblies. See WO2008132779A1 (US20100096905A1) for instance. Each drive assembly comprises a drive disk coaxially opposing the drive disk of the other drive assembly and a plurality of drive rollers obliquely arranged along the circumference of the drive disk at a regular interval so as to be individually rotatable. The main wheel comprises a ring-shaped annular member rotatably supported by a frame around a central axial line thereof and a plurality of driven rollers arranged along the circumference of the annular member so as to be rotatable around the respective tangential lines. As the drive disks are turned by the electric motors, the driven rollers are frictionally driven by the drive rollers. When the drive rollers are turned around the tangential directions of the main wheel, the vehicle is driven in a lateral direction. When the main wheel is turned around the central axial line thereof, the vehicle is driven in a fore and aft direction. The direction of motion of the vehicle can be selected as desired by suitably adjusting the difference between the rotational speeds of the two drive disks.
In such a vehicle, the force that the drive rollers apply to the driven rollers includes a component that turns the main wheel carrying the driven rollers around the central axial line of the main wheel and a component that turns the individual driven rollers around their respective central lines. The ratio of these two components of force is determined by the angular relationship of each drive roller with respect to the axial center line of the main wheel. This angular relationship characterizes the skewed relationship between the axial center line of the main wheel and axial center line of each drive wheel. This angular relationship may be represented by the angle between the intersection line of the contact surface between two lines; one of the lines being the intersection line defined the contact surface between the rive roller and driven roller and the plane of rotation of the drive roller, and the other line being the central axial line of the driven roller projected onto the aforementioned contact plane. This angle is called as a skew angle θ in the following description.
The skew angle θ is zero when the speed vectors of the drive roller and driven roller at the contact surface between them is perpendicular to each other, and is 90 degrees when the two speed vectors are parallel to each other. The smaller the skew angle is, the smaller the ratio of the rotational speed of the driven roller to the difference in the rotational speed between the two drive disks becomes. Conversely, as the skew angle approaches the 90 degree angle, the greater the ratio becomes. If there is no slippage between the drive roller and driven roller, in theory, the ratio becomes infinite when the skew angle is 90 degrees. However, in reality, the maximum value of the ratio is reached when the angle is somewhat smaller than 90 degrees.
An inverted pendulum type vehicle using such a frictional drive device is enabled to travel in the fore and aft direction by the rotation of the main wheel around the central axial line thereof, and the drive efficiency at such a time can be optimized by setting the skew angle close to zero. The vehicle is enabled to travel laterally by the rotation of the individual drive rollers around the respective center lines thereof or the tangential lines of the main wheel, and the drive efficiency at such a time can be optimized by setting the skew angle close to the 90 degree angle.
Thus, the drive efficiency of a frictional drive device of the aforementioned type can be optimized by setting different skew angles depending on the drive mode of the frictional drive device. The same is true with an inverted pendulum type vehicle using such a frictional drive device for a drive unit thereof. The drive efficiency of the vehicle can be optimized by setting different skew angles depending on the drive mode of the vehicle.